1. Field of the Invention
This invention relates to analog picture signal converters for facsimile and the like apparatus and more particularly to those of the type capable of time-axis conversion of the picture signal output of a solid-state image sensor.
2. Description of the Prior Art
Solid-state image sensors, including a large number of photosensor elements arranged unidimensionally over a definite length and an analog shift register for taking out the electric charges stored in the respective sensor elements, are employed in high-speed facsimile transmitters, OCRs, etc., as they are capable of obtaining picture-signal segments, as produced by the optical line-by-line scanning of the original picture, by electronic scanning.
However where a solid-state image sensor is applied to a so-called medium-speed or low-speed facsimile system, the picture signal output of the image sensor needs to be subjected to time-axis conversion before it is sent into the circuit line because of certain characteristics inherent in such image sensor.
In other words, since the sensing region of a solid-state image sensor functions in a thermally non-equilibrium state to store therein electric charges as picture-signal elements, such picture-signal elements must be obscured by thermal noise unless the charges stored in the respective sensor elements are transferred to the analog shift register before the sensor attains a thermally equilibrium state. The period or cycle time of a gate signal for such charge transfer is usually of the length of several tens msec or less and, accordingly, the picture signal is fed with such length of period.
On the other hand, in a medium- or low-speed facsimile, the scanning period of the picture signal to be sent out into the circuit line is determined in the range of from 80 msec to 350 msec as required under the bandwidth, image resolution and other circuit line conditions and, therefore, the picture signal cannot be sent out into the circuit line in the form as outputted from the solid-state image sensor.
Under the circumstances, it is considered that, if for the purpose of time-axis conversion of the picture signal, use can be made of a charge transfer device (CTD) such as a bucker-brigade device (BBD) or a charge-coupled device (CCD) as an analog shift register, the time-axis conversion of the graduated picture signal will be effected much more economically and compared with the case in which a digital memory is employed.
In this connection, in the article entitled "New Model Medium-Speed Facsimile NEFAX-3500" (in Japanese), the NEC TECHNICAL JOURNAL, No. 126, pp. 30-33 (Dec. 25, 1978), it is disclosed that "the output of the image sensor is stored at high speed in an analog shift register and read out thereof at a speed corresponding to the main scanning frequency", suggesting a technique employing an analog memory device for time-axis conversion of the picture signal from an image sensor.
In the use of a CTD, however, in which the picture signal input is stored within a semiconductor substrate as a succession of electric charges which are sequentially transferred, it is required that, to improve the signal-to-noise ratio, such signal be outputted from the CCD as soon as possible before the latter attains a thermally equilibrium state, as with the case explained above in connection with a solid-state image sensor. This means that, in order to realize the desired delaying, expansion or compression of the signal, it is necessary to provide some means for enabling the signal processing to be effected while the influence thereon of thermal noise is still limited.
Further, in the CTD, usually there exist some variations in characteristic between its elements and at least a few of such elements exhibit a dark current spike of substantial magnitude, necessitating some measure obviate the influence of such spiky dark current.